Adds Direction enum and abstracts the slideUp, slideDown, slideLeft, and slideRight methods.

main.js

@@ -1,3 +1,11 @@
+const Direction = Object.freeze({
+    Up: { name: "up" },
+    Right: { name: "right" },
+    Down: { name: "down" },
+    Left: { name: "left" },
+    get values() { return Object.values(Direction); },
+    fromName(name) { return this.values.filter(i=>i.name == name)[0] || null; },
+});
 const globals = {
     grid: null,
 };
@@ -18,48 +26,45 @@ const btnRedraw_onClick = () => {
     redraw(globals.grid);
 }
 
-const btnUp_onClick = () => {
+const slideInDirection = (grid, direction) => {
-    const grid = globals.grid;
+    switch(direction) {
-    if (slideUp(grid)) {
+        case Direction.Up:
+            return slideUp(grid);
+        case Direction.Down:
+            return slideDown(grid);
+        case Direction.Left:
+            return slideLeft(grid);
+        case Direction.Right:
+            return slideRight(grid);
+        default:
+            return false;
+    }
+}
+
+const playMove = (grid, direction) => {
+    if (slideInDirection(grid, direction)) {
         addRandomTile();
         redraw(grid)
     } else {
-        alert('Unable to slide Up.');
+        alert(`Unable to slide ${direction.name}.`);
     }
     checkForEnd(grid);
 }
 
+const btnUp_onClick = () => {
+    playMove(globals.grid, Direction.Up);
+}
+
 const btnDown_onClick = () => {
-    const grid = globals.grid;
+    playMove(globals.grid, Direction.Down);
-    if (slideDown(grid)) {
-        addRandomTile();
-        redraw(grid)
-    } else {
-        alert('Unable to slide Down.');
-    }
-    checkForEnd(grid);
 }
 
 const btnLeft_onClick = () => {
-    const grid = globals.grid;
+    playMove(globals.grid, Direction.Left);
-    if (slideLeft(grid)) {
-        addRandomTile();
-        redraw(grid)
-    } else {
-        alert('Unable to slide Left.');
-    }
-    checkForEnd(grid);
 }
 
 const btnRight_onClick = () => {
-    const grid = globals.grid;
+    playMove(globals.grid, Direction.Right);
-    if (slideRight(grid)) {
-        addRandomTile();
-        redraw(grid)
-    } else {
-        alert('Unable to slide Right.');
-    }
-    checkForEnd(grid);
 }
 
 const drawGrid = (grid, element) => {